The present invention relates to a process for agglomerating particulate material and the products produced by such processes. The processes are particularly useful for agglomerating metallic ores and, most particularly, iron ore.
Processes for agglomerating particles, especially metallic particles, are known in the art. Such processes are described more fully in, e.g. Canadian Patent No. 890 342, issued Jan. 11, 1972. As disclosed in Canadian Patent No. 890 342, it is well known to mechanically agitate water-wet particles to promote the operation of cohesive forces which produces larger agglomerates of the particulate solids. The mechanical agitation may be produced by rolling or cascading motion as is achieved in balling drums, discs and cones. Another agglomeration method utilizes agitation induced by paddle type agitators, such as in pug mills.
As agglomeration proceeds, aggregates in the form of pellets, balls, or granules are formed. As the agglomerates are agitated, e.g. rolled or tumbled, particles are added to their surface as a continuous film. The growth of larger agglomerates is also attributed to coalescence of smaller particles and agglomerates. Sometimes the agglomerates are dusted with finely divided dry particles to minimize sticking problems or sprayed with liquid, e.g. water, if the mixture becomes too dry. When their size is sufficient, the agglomerates are removed from the agitating mechanism for further processing such as induration by heating to low temperatures and sintering at higher temperatures depending upon the utilitarian nature of the starting materials.
International Patent Publication WO 88/00232 discloses a binder for fuels (especially coal) comprised of guar gum. A small amount of citric acid may be optionally added to adjust the pH.
European Patent Application Publication No. 0 376 713 discloses a process for making pellets of particulate metal ore, particularly iron ore. The process comprises mixing a water-soluble polymer with the particulate metal ore and water and pelletizing the mixture. The water-soluble polymer may be of any typical type, e.g., natural, modified natural or synthetic. The mixture may optionally comprise a pelletizing aid which may be sodium citrate.
U.S. Pat. No. 4,288,245 discloses pelletization of metallic ores, especially iron ore, with carboxymethyl cellulose and the salt of a weak acid.
Australian Patent Specification 46544/85 discloses a pelletizing process for iron ore employing hydroxymethyl cellulose and an inorganic salt (e.g. sodium carbonate). Guar gum may be used as a carrier.
European Patent Application Publication No. 0 203 855 discloses a binder comprised of a polymer (especially a polyacrylamide-based polymer) and an inorganic salt such as sodium carbonate. According to this disclosure, the polymer-inorganic salt binder may be used for agglomeration of both xe2x80x9cmineral orexe2x80x9d and xe2x80x9ccoal dust and nonmetallic materialsxe2x80x9d.
U.S. Pat. Nos. 4,863,512 and 4,919,711 disclose iron ore binder compositions comprised of alkali metal salts of carboxymethyl cellulose and/or carboxymethyl hydroxyethyl cellulose and sodium tripolyphosphate. Incidentally, these U.S. patents mention that their binder compositions may contain additional polysaccharides, such as guar and hydroxypropyl guar and inorganic salts, such as sodium citrate and sodium carbonate.
Abstract 22,244Q, 1968, abstracting the U.S.S.R. inventor certificate RU 205982, published July, 1968, discloses a method of preparing mixtures of powders for the production of sintered ferrites. In that process boric acid and sodium carboxymethyl-cellulose are solubilized. Barium ferrite powder is mixed with 6% of the solution, compressed, dried and sintered.
Even in the face of such technical knowledge, there remains a need for economical binders with improved properties.
In one embodiment, the current invention is a process of agglomerating particulate material, said process comprising commingling said particulate material with a moistening effective amount of water, a binding effective amount of polymer and a binding effective amount of weak acid to produce a mixture and forming said mixture into agglomerates.
In another embodiment, the current invention is a process of agglomerating particulate material, said process comprising commingling said particulate material with (1) a moistening effective amount of water, (2) a binding effective amount of a polymer selected from the group consisting of guar, guar derivatives, starch, modified starch, starch derivatives, alginates, pectins and mixtures thereof and (3) a binding effective amount of the salt of a weak acid to produce a mixture and forming said mixture into agglomerates.
In yet another embodiment, the current invention is pellets comprised of particulate material, a binding effective amount of polymer and a binding effective amount of a weak acid. Optionally, the pellets may be comprised of a polymer selected from the group consisting of guar, guar derivatives, starch, modified starch, starch derivatives, alginates, pectins, and mixtures thereof and the salt of a weak acid.
Polymers. The polymers useful in the present invention may be (1) a water-soluble natural polymer, such as guar gum or starch, (2) a modified natural polymer, such as guar derivatives (e.g. hydroxypropyl guar, carboxymethyl guar), modified starch (e.g. anionic starch, cationic starch), starch derivatives (e.g. dextrin) and cellulose derivatives (e.g. hydroxyethyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose, methyl cellulose), and/or (3) a synthetic polymer (e.g. polyacrylamides, polyacrylates, polyethylene oxides). Such polymers may be used alone or as combinations of two or more different polymers.
The binding effective amount of polymer will vary depending upon numerous factors known to the skilled artisan. Such factors include, but are not limited to, the type of particulate material to be agglomerated or pelletized, the moisture content of the particulate material, particle size, the agglomeration equipment utilized, and the desired properties of the final product, e.g. dry strength (crush), drop number, pellet size and smoothness. Though not limiting, a binding effective amount of polymer will typically be in the range of about 10 to about 99 wt. % and about 40 to about 95 wt. % based on total binder weight.
Acids and Their Salts. The acids useful in the current invention are weak organic or inorganic acids, having degrees of acidity such that their pK is higher than about 3. The pK is defined here as pK=xe2x88x92log K, where K is the dissociation constant of the acid or already dissociated acids at 25xc2x0 C. in water (see C. D. Hodgeman, Handbook of Chemistry and Physics, 30th Ed., 1947, p. 1425). As non-limiting examples of such acids may be mentioned: acetic acid, benzoic acid, lactic acid, propionic acid, tartaric acid, succinic acid, citric acid, nitrous acid, boric acid, carbonic acid, fumaric acid, malic acid and the like.
In certain embodiments of the current invention, use is made from the salts derived from such acids and, for example, alkali metals (e.g. sodium, potassium and lithium,) ammonia, etc. Particularly preferred salts are those derived from alkali metal and citric and or carbonic acid, such as carbonates and bicarbonates and citrates of potassium and sodium. The salts contemplated herein may be used in their hydrated or anhydrous forms. Specific salts of interest are sodium citrate, sodium carbonate, sodium tartrate, sodium bicarbonate, sodium stearate, sodium benzoate, sodium oxalate, sodium acetate, sodium glycolate and the corresponding ammonium, potassium, calcium and magnesium salts of these acids.
A binding effective amount of weak acid or salt of a weak acid, as with the polymer, will depend on many factors well known to the skilled artisan. However, generally, a binding effective amount of weak acid or salt of a weak acid will be about 1 to about 90 wt. % acid and preferably about 5 to about 60 wt. % based on total binder weight.
Binder Addition. The amount of binder, comprised of polymer and weak acid or salt of a weak acid, added to particulate material to be agglomerated will depend on many factors as discussed above. However, a typically effective amount of binder added is 0.01 to about 5.0 wt. %, and preferably about 0.03 to about 0.3 wt. %, of the agglomerating mixture.
The binder may be added in any of the typical physical forms as known by the skilled artisan, e.g. dry, liquid, emulsion, dispersion, etc.
Water. The initial moisture content of the particulate material, polymer and acid or weak acid salt mixture will also depend on many factors known to the skilled artisan. As non-limiting ranges, generally, the water content of such mixture should be about 4 to about 30 wt. % based on the weight of dry particulate matter and most preferably about 7 to about 12 wt. %.